1. Field of the Invention
The present invention relates to catalysts used in petroleum refining, and particularly to a multiple zeolite catalyst used to convert heavy aromatic hydrocarbons, principally C9+ aromatics, to BTX (benzene, toluene, and xylene), and particularly to commercially valuable xylene isomers.
2. Description of the Related Art
Aromatic hydrocarbons are the building blocks for many industrially important products. They are generally produced in a petrochemical complex. There are several commercial processes producing aromatics especially xylenes isomers, using a variety of reactions. Xylene isomers, para-xylene, meta-xylene and ortho-xylene, are important intermediates, which find wide and varied application in chemical syntheses. Upon oxidation, p-xylene yields terephthalic acid, which is used in the manufacture of polyester plastics and synthetic textile fibers (such as Dacron), films (such as Mylar), and resins (such as polyethylene terephthalate, used in making plastic bottles). m-Xylene is used in the manufacture of plasticizers, azo dyes, wood preservers, etc. o-Xylene is feedstock for phthalic anhydride production, which is used to make polyester, alkyd resins, and PVC plasticizers.
Xylene isomer streams from catalytic reforming or other sources generally do not match demand proportions as chemical intermediates. p-Xylene, in particular, is a major chemical intermediate with rapidly growing demand, but amounts to only 20 to 25% of a typical C8 aromatics stream. Among the aromatic hydrocarbons, the overall importance of the xylenes rivals that of benzene as a feedstock for industrial chemicals. The xylenes are produced from petroleum by the reforming of naphtha in insufficient volume that is difficult to meet the demand, and conversion of other hydrocarbons is necessary to increase the yield of xylenes.
A current objective of many aromatics production facilities is to increase the yield of xylenes by converting heavy aromatics, such as C9, C10 and C11+, and to de-emphasize benzene production. Demand is growing faster for xylene derivatives than for benzene derivatives. Refinery modifications are being effected to reduce the benzene content of gasoline in industrialized countries, which will increase the supply of benzene available to meet demand. A higher yield of xylenes at the expense of benzene, thus, is a favorable objective, and processes to convert C9+ aromatics have been commercialized to obtain high xylene yields.
Aromatic hydrocarbon compounds contained in a gasoline base generally have higher octane values and are superior as a gasoline base because of their high calorific value. Among them, toluene and aromatic hydrocarbon compounds, those having eight carbon atoms especially, have higher octane values and driveability levels; thus, it is desirable to increase the volume of C8 aromatic compounds in gasoline. In particular, methods of directly converting aromatic hydrocarbon compounds having nine or more carbon atoms in a gasoline fraction into toluene and aromatic hydrocarbon compounds having eight carbon atoms are significantly meaningful.
Reactions of aromatic hydrocarbon compounds to convert aromatic hydrocarbon compounds to compounds having a different number of carbon atoms include the transalkylation reaction and the disproportionation reaction. A transalkylation reaction is one in which an alkyl group, e.g., a methyl group, is detached from a first compound and then attached to a second compound. A disproportionation reaction is a reaction in which a single compound acts as both an oxidizing agent and a reducing agent.
A well known process regarding these reactions is the manufacture of xylenes utilizing the disproportionation reaction of toluene, i.e., two molecules of toluene react to form one molecule of benzene and one molecule of xylene (by transfer of a methyl group from one molecule of toluene to the other, a transalkylation reaction). Transalkylation reactions, however, are not limited to the disproportionation of toluene. Other methods of increasing xylene yields operate through inducing transalkylation by adding aromatic hydrocarbon compounds having nine or more carbon atoms into the starting materials, resulting in such reactions as the addition of one mole of toluene to one mole of a C9 aromatic hydrocarbon to produce two moles of xylene. Examples of such transalkylation reactions are illustrated in paragraphs [0009] through [0011] of U.S. Patent Publication 2005/0187518, which are hereby incorporated by reference.
Further, it is known to separate isomers through molecular sieves formed by zeolites. Zeolites are generally hydrated aluminum and calcium (or sodium) silicates that can be made or selected with a controlled porosity for catalytic cracking in petroleum refineries, and may be natural or synthetic. The pores may form sites for catalytic reactions to occur, and may also form channels that are selective for the passage of certain isomers to the exclusion of others. Zeolites may serve as Brönsted acids by hydrogen ion exchange by washing with acids, or as Lewis acids by heating to eliminate water from the Brönsted sites. For example, the zeolite ZSM-5 (Na3Al3Si93O192.16H2O) has a pore size that results in the formation of channels of such size and shape that it forms a selective sieve for xylene isomers. The alkylation of toluene by methanol will form a mixture of all three xylene isomers. p-Xylene will pass through the channels in ZSM-5 due to its linear configuration, while o-xylene and m-xylene will not pass through the pores, although they may subsequently rearrange to p-xylene under the acidic conditions in the pores and then pass through the sieve. See Huheey et al., Inorganic chemistry, 4th ed., pp. 745-748.
The catalytic activity of zeolites can also be increased by addition of a metal catalyst that activates hydrogen by breaking up molecular hydrogen to atomic hydrogen on the surface of the metal for forming intermediates in transalkylation reactions.
Many types of supports and elements have been disclosed for use as catalysts in processes to convert heavier aromatics into xylenes. However, as the number of such supports and elements attests, none have been found entirely satisfactory. Hence, an improvement of even a few percentage points in conversion efficiency may be significant, particularly when practiced at high volumes on an industrial scale in oil refining facilities. Thus, a multiple zeolite catalyst solving the aforementioned problems is desired.